Lightweight concrete roof slope should be evaluated as a geometry problem across the whole roof area. The important question is how proposed elevations connect to existing drains, walls, curbs and thresholds. Adding material in one location does not by itself prove that water will have an appropriate route off the roof.
Start with a location-based drawing
Ask the project team to record existing drainage points and significant elevation changes. Mark recurring wet areas on the same plan, using photographs taken from safe, authorized locations. Describe when those observations occurred. A dry-weather photograph alone may not show where water collects after rainfall.
GAF's concrete-deck overview identifies lightweight insulating concrete as a distinct assembly component over supporting systems. When that component is involved in a drainage proposal, ask the designer to explain its intended role and limits in the particular roof. Do not assume every concrete layer can be altered in the same way.
Check the interfaces before accepting the slope plan
Review where the proposed roof surface meets doors, equipment curbs and wall flashings. These fixed features can constrain a design. Ask whether any must be modified and whether the price includes that work. A plan that improves one low area while overlooking a threshold can create an incomplete project scope.
Consider a hypothetical service roof with a drain near a doorway and equipment between them. The owner should request an elevation drawing showing the intended path around those obstructions. “Add slope toward the drain” is too general to explain how the transitions will actually fit together.
Define how the finished work is documented
Ask what records will confirm the constructed elevations and how deviations from the design are reviewed. Preserve the final drawing, photographs of concealed work and any approved changes to drainage components. If the project depends on plumbing or structural work, identify the responsible trade and reviewer explicitly.
Also clarify the maintenance tasks that remain after construction. A redesigned surface does not eliminate the need to monitor debris and keep authorized access available. The owner should know who to contact if observed drainage differs from the project's documented intent.
Perkins' flat-roof overview provides background on low-slope assemblies, and the commercial inspection checklist helps prepare observations. Use those observations to request a coordinated design and scope, rather than treating a material addition as a universal solution to every ponding complaint.
Manufacturer references: GAF: Concrete roof deck types.